To explore the use of a CaO-based sorbent to capture CO2 from flue crases of combustors in multiple absorption/desorption cycles, formulas for the decay behavior of the absorption capacity have been examined and a new simple equation has been proposed. This equation is shown to describe the reported data better and uses only one parameter. By analogy with the deactivation of catalysts by sintering, it is suggested that sintering of the sorbents is the cause of the capacity decay. With the proposed equation, the activities of different sorbents can be compared using the value of a single parameter, which describes the rate of decay of activity.